Abstract

A collisionless field‐free synthesized Cs+ plasma stream is impacted on the transverse magnetic field of a C‐electromagnet. The configuration of a ``narrow'' beam is maintained as stream widths (∼4 cm) are small compared to the ion cyclotron radius. The interaction is observed to be unstable when the plasma penetrates or traverses a magnetic field in excess of ∼5 G. The instability is manifested by the appearance of both high (10–100 Mcps) and low (100 kcps) oscillations. It is believed that the instability arises from contrastreaming electron currents generated by a peaked potential structure in the boundary gradient region.